Innate Immunity: The Body's First Lines of Defense

Innate Immunity

Overview of Innate and Adaptive Immunity

The immune system protects the body from pathogens through two main branches: innate immunity and adaptive immunity. These systems differ in their response time, specificity, and memory.

• Innate Immunity: The body's immediate, non-specific defense mechanism present in all eukaryotes. It does not have memory and responds the same way to repeated exposures.

• Adaptive Immunity: A slower, highly specific response found only in vertebrates. It develops memory after exposure, leading to a faster and stronger response upon re-exposure to the same pathogen.

Feature	Innate Immunity	Adaptive Immunity
Response time	Immediate	4–7 days
Presence in organisms	All eukaryotes	Only vertebrates
Antigen specificity	No	Yes
Memory	No	Yes

Three Lines of Immune Defense

1. First-Line (Barrier) Defenses

First-line defenses are physical, mechanical, and chemical barriers that prevent pathogen entry.

• Mechanical Barriers: Physically remove pathogens through actions such as flushing, rinsing, or trapping. Examples: Tears, mucus, cilia in the respiratory tract.

• Chemical Barriers: Substances that destroy or inhibit pathogens. Examples: Lysozyme in saliva and tears, antimicrobial peptides, acidic pH of the stomach.

• Physical Barriers: Structural features that block pathogen entry. Examples: Skin, tightly packed epithelial cells lining mucous membranes.

2. Second-Line Defenses

These defenses are activated if pathogens bypass the first line. They include cellular and molecular components.

• Cellular Defenses: Various leukocytes (white blood cells) that identify and eliminate pathogens.

• Molecular Defenses: Soluble factors such as cytokines, the complement system, and iron-binding proteins that enhance immune responses.

3. Third-Line (Adaptive) Defenses

The third line of defense is the adaptive immune response, which is antigen-specific and forms immunological memory. Note: This is covered in detail in Chapter 12.

The Lymphatic System in Immunity

The lymphatic system is a network that transports lymph, filters pathogens, and facilitates immune cell movement. It is essential for both innate and adaptive immunity.

• Lymph Nodes: Main filtering centers where immune responses are initiated.

• Spleen: Filters blood and helps mount immune responses to blood-borne pathogens.

• MALT (Mucosa-Associated Lymphoid Tissue): Protects mucosal surfaces.

• Thymus: Site of T cell maturation.

• Bone Marrow: Produces all blood cells, including immune cells.

Leukocytes (White Blood Cells) in Innate Immunity

Leukocytes are the primary cellular components of the immune system. They are classified as granulocytes or agranulocytes based on the presence of granules in their cytoplasm.

• Granulocytes:

  • Neutrophils: Most abundant; phagocytose bacteria and debris.

  • Eosinophils: Combat parasitic infections; involved in allergic responses.

  • Basophils: Release histamine; involved in allergy and parasite defense.

  • Mast Cells: Tissue-resident; release histamine in allergic reactions.

• Agranulocytes:

  • Monocytes: Circulate in blood; differentiate into macrophages for phagocytosis.

  • Dendritic Cells: Antigen-presenting cells that bridge innate and adaptive immunity.

  • Lymphocytes: Include NK (natural killer) cells (innate immunity), and B and T cells (adaptive immunity; covered in next chapter).

Leukocyte differentials (proportions of different WBC types) are important clinical diagnostics. For example, increased neutrophils (neutrophilia) often indicate bacterial infection.

Molecular Defenses in Innate Immunity

Molecular defenses include proteins and small molecules that enhance immune responses and inhibit pathogen growth.

• Cytokines: Signaling proteins that regulate immune responses.

  • Chemokines: Attract white blood cells to infection sites.

  • Interleukins: Mediate communication between immune cells; regulate inflammation and immune cell development.

  • Interferons: Induce antiviral states in cells; enhance immune responses.

  • Tumor Necrosis Factors (TNFs): Promote inflammation and can induce cell death.

• Iron-Binding Proteins: Restrict pathogen access to iron, an essential nutrient. Examples: Transferrin, lactoferrin.

• Complement System: A cascade of proteins activated by three pathways (classical, alternative, lectin) leading to:

  • Opsonization: Coating pathogens to enhance phagocytosis.

  • Cytolysis: Direct lysis of pathogens.

  • Inflammation: Recruitment of immune cells and increased vascular permeability.

Inflammation and Fever

Inflammation

Inflammation is a localized tissue response to injury or infection, characterized by three phases:

1. Vascular Changes: Increased blood flow and vessel permeability.

2. Leukocyte Recruitment: Migration of immune cells to the affected area.

3. Resolution: Elimination of pathogens and initiation of tissue repair.

Main goals: Recruit immune defenses, limit pathogen spread, and promote healing.

Cardinal signs: Redness, pain, heat, swelling, and loss of function.

Fever

Fever is a systemic increase in body temperature, triggered by pyrogens (fever-inducing substances). It enhances immune functions such as phagocytosis, interferon activity, and tissue repair.

• Low-grade fever: Generally protective and beneficial.

• High fever: Can be dangerous and requires medical attention.

Example: Complement Activation Pathways

• Classical Pathway: Triggered by antibodies bound to pathogens.

• Alternative Pathway: Activated directly by pathogen surfaces.

• Lectin Pathway: Initiated by mannose-binding lectin binding to pathogen carbohydrates.

All pathways converge to activate C3 convertase, leading to pathogen destruction.

Additional info: The adaptive immune system (third line of defense) will be discussed in detail in the next chapter, including the roles of B and T lymphocytes, antibody production, and immunological memory.